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Giant Moth Honored As Research Heavyweight

By Margrit McIntosh,
May 30, 2001

UA scientists met on March 2nd to share their knowledge of a large beautiful moth that has become a model study organism: Manduca sexta, the tobacco hornworm moth.

This moth has been a focus of neurobiological and biochemical study for at least 30 years, and for about 20 years at UA. Recently the moth has also been adopted as a study organism by researchers in such diverse fields as physiology, evolution, chemical ecology, and behavior. Scientific publications related to Manduca sexta have jumped from 15 papers in 1960-1969, to 2,846 in 1990-1999 .

Why have scientists flocked to Manduca sexta as a study organism? "It's like the (Boeing) 777 of moths," explains UA Regents' Professor John Hildebrand, a long-time researcher of M. sexta, referring to the large size of this insect (adult moths have a wingspan of 3 1/4 to 4 1/4 inches). Large size makes it easy for researchers to perform delicate dissections.

"We can do things routinely that can't be done in flies," explains Hildebrand, such as recording electrical impulses from identified individual cells. Studies of the neurobiology of this insect by John Hildebrand and Tom Christensenand others recently led to a paper in the March 22nd issue of Nature.

The second big advantage of M. sexta as a study organism is that, because of its life cycle, many developmental events are delayed until the insect is at adult size. In addition, M. sexta can easily be reared in the lab, and it goes through several broods a year. This means researchers can be assured of a constant supply of healthy animals.

Organized by Goggy Davidowitz, a postdoctoral researcher at UA, the Manduca Symposium attendees shared their expertise, successes and failures, and even gave tribute to the moth in the form of a poem by UA Regents' Professor Elizabeth Bernays . Symposium topics ranged from the physiology and neurobiology of olfaction (sense of smell), to immune responses of the moth to natural enemies, to host plant selection.

At the Manduca symposium, Goggy Davidowitz presented preliminary results from a study of body size in Manduca sexta - how is it that this insect is the size that it is? An insect's size is determined, in part, by the size of the larvae when it starts the sequence of events that lead to pupation. This sequence is initiated once the larva reaches a "critical weight." Larger is better as larger females generally produce more offspring. Davidowitz is studying how this critical weight changes with diet quality and temperature to determine how it is regulated. This in turn can lead to an understanding of how body size evolves and ultimately what makes an insect the size that it is.

Mark Willis showed video of a moth-sized wheeled robot navigating its way towards an "odor" source. The robot, developed in collaboration with Wayne Jouse in Aerospace and Mechanical Engineering, has two "antennae" which it uses to detect ionized air (a surrogate for airborne odors), and sensors for identifying wind direction. Its attempts to navigate toward the source are guided by "rules for navigation" developed in modeling studies that attempt to simulate moth odor-tracking behavior.

Alejandro Mira presented results from a study of host plant use in local populations of Manduca sexta. Locally the caterpillars of these moths are found on datura, and also on devil's claw. Mira found that fewer eggs hatch on devil's claw than on datura, but those that do hatch are much more likely to survive, because fewer predators and parasitoids attack caterpillars on devil's claw than on datura. This "tradeoff" between hatching success and larval survival is a classic ecological phenomenon.

Ann Fraser combined two methods for studying host plant odors and how the moth reacts to them: the electroantennogram (EAG), in which an electrode is inserted into the moth's antennae, and nerve impulses recorded, and the gas chromatograph (GC), which chemically analyzes odors. She caused plant odors to flow over a moth antenna hooked up to an EAG, and simultaneously chemically analyze those compounds in a GC. Using this technique, Fraser is starting to characterize the complex odors of tomato plants into single compounds, seeing how the moth responds to those compounds, and determining the roles they play in hostplant selection.

"It's great that there's a whole little community that's developed ... what started out as a model for cellular, developmental events has now become also a great model for studying insect-plant interactions," commented Hildebrand.

This same moth is the focus of a UA outreach project to K-12 students,

"The Manduca Project," which helps students rear and study the moths, learning about biology and the scientific method in the process.

The participants felt the Manduca Symposium was such a success that they voted to make it an annual event.